We are studying the properties and structure of the outer membrane of E. coli. A mutant showing increased permeability to a large number of hydrophobic compounds was found before to have altered lipopolysaccharide and no alteration in other outer membrane components. We now have shown that the altered lipopolysaccharide increases the fluorescence of hydrophobic tetracycline derivatives more than normal lipopolysaccharide, suggesting that the altered lipopolysaccharide permits increased permeability by increasing partition of hydrophobic molecules into the membrane. We have also devised a gel electrophoresis method that shows far more heterogeneity in lipopolysaccharide from a single strain than heretofore detected. In other experiments we find that the small amount of protein that copurifies with lipopolysaccharide in butanol "endotoxin" preparations is all outer membrane protein which is of interest because of the lymphocyte mitogenicity of this protein. In studies with macrophage cell lines, we have selected mutants that do not show generalized phagocytosis. We also have defined by gel electrophoresis several external membrane proteins that are reduced by borotritide after sialic acid oxidation, and that completely lose their label on sialidase treatment. These will be used as markers in studies of membrane dysfunction.